Plato is one of the most engaging writers in the Western literature and certainly one of the most influential authors in the history of philosophy. The son of wealthy and influential Athenian parents, Plato was well-placed to participate in the political and philosophical life of ancient Athens and his works detail the political events and intellectual movements of his time. However, the questions he raised in his writing are so profound and the strategies he uses for resolving them so persuasive that they have influenced educated readers in every era since. 
One of the unique aspects of Plato is his self-consciousness and his concept of how philosophy should be perceived, and its scope and ambitions. He reasoned that philosophy should be a rigorous and systematic examination of ethical, political, metaphysical, and epistemological issues. He also enjoys a unique position in the pantheon of great philosophers as he was both a student of Socrates and a teacher of Aristotle.
Plato began his philosophical career as a student of Socrates and it is through Plato’s writings that we get most of our information about Socrates’ ideas. After Sophocles was tried and committed suicide, Plato traveled to Egypt and Italy and studied with students of Pythagoras. Eventually, he returned to Athens and established his own school of philosophy called The Academy.
In his earliest works, Plato presented Socrates' teaching in the form of dialogues in which Socrates is presented as the chief teacher and debater. These early dialogues usually investigated a single issue -- such as what is friendship, what is virtue, can virtue be taught? – however, a conclusive result was rarely achieved. The Euthyphro raised doubts about the relationship of morally correct actions and divine authority. The Apology presented Socrates’ defence in front of an Athenian jury and offers a description of the philosophical life. Finally, The Crito describes Socrates's imprisonment and asks whether an individual citizen is ever justified in refusing to obey the state.
Socrates continues as a fictional character in Plato’s middle dialogues such as The Meno and The Phaedo, although the ideas are clearly those of Plato.

The Republic

Plato’s masterpiece is The Republic. It is one of the most influential works in Western philosophy. It deals with the central problem of how to live a good life; and tackles the parallel questions of what is justice, what would an ideal state be like, and what is a just individual? It begins with a Socratic conversation about the nature of justice and continues with a discussion on the virtues of wisdom, courage, and moderation as they appear both in individual human beings and in society as a whole. This plan for the ideal society or person requires detailed accounts of human knowledge and of the kind of educational program by which it may be achieved by men and women alike. The dialogue concludes with a review of various forms of government, an explicit description of the ideal state, in which only philosophers are fit to rule, and an attempt to show that justice is better than injustice.
One important aspect of Plato's thought is the theory of "ideas" or "forms." Plato was concerned by the question of change in the physical world. Heraclitus had said that there is nothing certain or stable except the fact that things change, while other philosophers claimed that all change, motion, and time was an illusion. Where was the truth? How can these two opposite positions be reconciled? Plato ingeniously combined the two in the Allegory of the Cave and the Dividing Line. Plato imagines two worlds, the visible world and the intelligible world, each existing on a line that can be divided in the middle: the lower part of the line consists of the visible world and the upper part of the line makes up the intelligible world. Each half of the line relates to a certain type of knowledge: of the visible world, we can only have opinion; of the intelligible world we achieve "knowledge". Each of these divisions can also be divided in two. The visible or changing world can be divided into a lower region of "illusion," which is made up of shadows, reflections, paintings, poetry, etc., and an upper region of "belief," which refers to any kind of knowledge of things that change. "Belief" may be true some or most of the time but occasionally is wrong; belief is practical and may serve as a relatively reliable guide to life but doesn't really involve thinking things out to the point of certainty. The upper region can be divided into, on the lower end, "reason," which is knowledge of things like mathematics but which require that some postulates be accepted without question, and "intelligence," which is the knowledge of the highest and most abstract categories of things, an understanding of the ultimate good.